Synthesis and crystal structure of the rubidium scandium telluride RbSc5Te8

During attempts to synthesize the rubidium dicopper triscandium hexatelluride RbCu2Sc3Te6 in analogy to CsCu2Sc3Te6 from 2:3:6-molar mixtures of the elements (Cu, Sc and Te) with an excess of RbBr as flux and rubidium source, after 14 days at 900 degrees C in torch-sealed evacuated silica tubes brown lath-shaped crystals of RbSc5Te8 did form instead. This new compound crystallizes monoclinically in space group C2/m (no. 12) with two formula units in a unit cell of the dimensions a = 2130.61(9) pm, b = 413.94(2) pm, c = 1022.03(5) pm and beta = 104.392(4)degrees. The crystal structure of RbSc5Te8 consists of a three-dimensional anionic framework (3)(infinity){[Sc5Te8](-)} of face-, edge- and vertex-sharing [ScTe6](9-) octahedra that provides one-dimensional tunnels with a distorted square shape. For charge compensation they are occupied with Rb+ cations (CN = 10) coordinated in a trans-face bicapped cubic fashion by Te2- anions.